Method and apparatus for ultrasound scanning

ABSTRACT

A method and apparatus for compensating for the effects of variable attenuation in a body being scanned by an ultrasound pulse echo scanner in which a compensation signal is derived on the basis that the signal strength of the received signal at any time instant after generation is influenced by both attenuation and back scatter. The apparatus comprises an array of transducers which generate a received signal from an echo, for applying a time-dependent gain to produce a signal which is a function of the received signal, temporary storage and integrator functions operable to sum the signal values for each time instant from a commencement point, the combining of the integrated signal with the stored original signal to form a quotient, and the displaying of the signal thus generated.

BACKGROUND OF THE INVENTION

The present invention relates generally to ultrasound scanning.

Images produced by transmission of ultrasonic pulses through tissue, andgenerated from reflected echoes can be directly displayed in real time.Ultrasonic scanning constitutes a valuable non-invasive diagnostic toolfor gaining information concerning the interior structure of a body. Inmedical applications, where the "body" may be a living human or animalbody, inhomogeneity of the tissue causes different degrees ofattenuation and backscatter of the ultrasonic signal which result in amedically unsatisfactory displayed image.

Known ultrasonic scanners typically have a transducer (which may inpractice comprise a plurality of individual transducer elements in alinear or annular array), operated to transmit and receive ultrasonicpulse signals. Ultrasonic pulses transmitted from an individualtransducer element into the body, reflected at points therein andreceived by the same transducer element provide information on theinterior structure of the body in a line extending into the body fromthe transducer element. Such a one-dimensional line of information isreferred to as an A-scan. A two-dimensional image, referred to as aB-scan can be generated by scanning a transducer generating successiveA-scan lines. Such scanning may be achieved by physical displacement ofan individual transducer (that is a transducer comprising a singletransducer element) or by electronic scanning in the case of atransducer comprising an array of individual transducer elements. AB-scan image therefore represents a section through the body passingthrough the line defined by the transducer (if it comprises an array oftransducer elements) or by the path traversed by an individualtransducer, which, upon display on for example a video monitor screen,can be used to visualise the interior structure of the body.

In order to normalize the display, and to take account of the fact thatin a homogeneous body the echo signals from points deeper into the bodywill be more strongly attenuated due to the greater path length whichthey have travelled, it is normal to subject the received signals to anamplification with a time-dependent gain. The gain function of suchknown instruments, which effectively varies the gain as a function ofdepth, is based on average material properties. In a real lifesituation, however, where there are localized regions (such as cartilageor amniotic fluid) which may have greater or lesser attenuation than theassumed average, the time-dependent gain variation function isincorrect. Thus, although some compensation may be obtained byprogressively increasing the receiver gain for signals from greaterranges it must be accepted that no simple compensation function mayaccommodate the possible variations in tissue structure and that thenormal practice of applying the same gain function repeatedly is acompromise which, while it offers a degree of improvement over theuncorrected signal, is still far from perfect.

Although known scanners are provided with means for tuning or varyingthe gain function slightly in order to optimize the image, in otherwords to take account of the physical features within the actual bodybeing examined, even when they are adjusted to obtain the best resultsthe image includes image artifacts in the form of shadows, namely areaswhich are darker, in relation to the surrounding image, than the generallevel of the image, and "enhancements" or "inverse shadows", which areareas of greater brightness than the correct brightness for that area.It is observed that the "shadows" appear behind physical features whichattenuate the energy in the transmitted ultrasonic pulse more strongly,such as cartilage tissue, whereas the "enhancements" appear behind (thatis more deeply within the body in the direction of the section from thesurface) physical features which attenuate the energy less strongly,such as cysts or amniotic fluid.

It will be appreciated that the image presented to the operator byultrasonic scanning equipment is principally the result of two physicalphenomena related to the transmission of sound through solids andliquids constituting the body under examination. The first of thephysical phenomena is the attenuation of the sound energy as it istransmitted through the material itself, and the other is thebackscatter or reflection of the sound energy at an interface betweentwo materials having different properties. Backscatter is the principalfactor influencing the arrival of the echo signal since, unless it werereflected, the pulse would continue in the same direction and no echosignal would be received, whereas the attenuation phenomenon stronglyinfluences the resultant amplitude of the received signal. There areother factors which influence the received signal. These include theeffects of interference due to the coherent nature of sound waves, whichresults in a "speckle" appearing in the final image; refraction due tovariations in the velocity of sound at an interface between thedifferent tissues, which can cause distortion in the image or multipleimages; and the variation in the attenuation and backscatter of sound independence on the frequency of the sound signal. Ultrasonic pulsesgenerated by most existing scanners are composed of a range of frequencycomponents which results in the pulse shape being altered due to thedifferent behaviour of different frequency components. These, however,are second-order effects in comparison with the attenuation andbackscatter.

In known machines it is assumed that the backscatter and attenuation areindependent from one another. On this assumption the time-varying gainapplied to the received signal in order to compensate for the effects ofattenuation is the same for all A-scan lines regardless of location. Itis believed, however, that the backscatter and attenuation are notentirely independent from one another and that the application of anunmodified or uncompensated time-varying gain to the signal is, in fact,a major reason for the appearance of the shadowing and enhancementeffects referred to above.

Attempts have been made to improve the imaging signal over that using aconventional time gain control. One of these attempts involves examiningpeak signals in a number of range segments, and anticipating therequired gains from a progressively updated store. In this known system,detector output is sensed in an integrated peak detector rather than asmoothing filter and the peak value derived in each segment of the totalrange is held in a corresponding storage capacitor. After some number ofcomplete image formations these capacitors will hold a voltage derivedfrom the complete set of signals in a given range segment of the imagedslice. The gain control function is then derived by reading out thevalue stored for each range segment, modified by simultaneouslyweighting a portion of the value for the next segment so that thecontrol is weighted by the echo history over the total spatial extent ofthe range segment, by the previous range behaviour held by a filter, andby the anticipated behaviour held in the next storage capacitor. This,however, takes several seconds, and therefore cannot be used in a realtime display instrument.

Because the same time-varying gain is applied to each A-scan line makingup a two-dimensional B-scan image, no allowance can be made forvariation in attenuation properties in the direction perpendicular tothe A-scan lines. However, the attenuation in a body is not necessarilyconstant so that application of a predetermined time gain functionrepresents a compromise necessary due to the fact that the variation inattenuation is an unknown.

The present invention seeks to provide a method and means forcompensating the effects of variable attenuation in a body being scannedby an ultrasound pulse echo scanner, and in particular a method andmeans by which the shadowing and enhancement appearing in the imagesproduced by conventional pulse echo scanners can be reduced if notentirely eliminated.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, therefore, a method ofcompensating for the effects of variable attenuation in a body beingscanned by an ultrasound pulse echo scanner comprises the generation ofa compensation signal derived from the received signal by determining,for any time instant in the received signal, the sum of the values of afunction of the received signal, over a time interval commencing withthe said time instant, and generating a display signal which is thequotient of a function of the received signal and the compensatingsignal.

The function of the received signal may, of course, be the identityfunction.

The theoretical considerations underlying this method will be discussedin more detail below. In practice, the said received signal may betemporarily stored while the values of the said compensation signal arebeing determined.

The compensation signal is derived on the basis that the signal strengthof the received signal at any time instant after generation isinfluenced by both attenuation and backscatter, and that there is alinear or non-linear relation between backscatter and attenuation in themedium through which the pulse is transmitted.

It is this realization that a relationship exists between backscatterand attenuation which underlies the method and apparatus of the presentinvention.

According to another aspect of the invention apparatus for producingultrasound images of a body under examination comprises a transducermeans for generating a pulse signal to be transmitted by the saidtransducer, and received thereby as an echo signal after reflectionwithin the body under examination, to generate a received signal forproducing an A-scan which, in combination with signals from other suchA-scan lines forms a B-scan of the body under examination, in whichthere are provided means by which image artifacts in the B-scan signalare at least reduced by combination of a function of the received signalwith a compensation signal derived by integration of a function of thereceived signal over a finite time period to form a quotient signal forapplication to display means for displaying the image thus produced.

The apparatus of the present invention may include temporary storagemeans for storing the said received signal, integration means forintegrating the received signal over a finite time period, and dividermeans for dividing the said received signal by the said integratedcompensation signal to produce the output signal for application to thesaid display means.

The present invention also comprehends apparatus for use in associationwith an existing ultrasound scanner for developing a compensated B-scansignal from a signal extracted from the scanner itself.

Embodiments of the present invention will now be more particularlydescribed, by way of example, with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram representing the variation of a received ultrasonicsignal with time;

FIG. 2 is a diagram illustrating the formation of the shadowing effect;

FIG. 3 is a diagram illustrating the formation of the enhancementeffect;

FIG. 4 is a block diagram illustrating the major components in acompensation system formed as an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, consider a coordinate system set up so that a transducer at theorigin transmits a sound pulse along the x axis at the velocity ofsound, c. A sound pulse transmitted from the transducer travels in thepositive x direction. In travelling from the point x to the point x+δxthe signal is attenuated by the tissue between these two points. If δxis assumed to be small it can be assumed that the properties of thetissue remain constant in this small region. The attenuation in thesignal strength as the pulse passes between the point x and the pointx+δx is expected to be proportional to the distance δx. It will alsodepend on the attenuation properties of the tissue represented as thequantity a(x) which represents the fraction of the signal which isremoved, per unit distance, during transmission between x and x+δx. Thepossible values of a(x) thus lie between 0 and 1.

Denoting the amplitude of the pulse when it gets to the point x by S(x),

    S(x+δx)=(1-a(x)δx)S(x)

Dividing by δx and taking limits as δx→0 gives ##EQU1## which can besolved to give ##EQU2## which relates the amplitude at the point x tothe amplitude at some other point, x₀.

At any point x some fraction of the signal will be scattered in adirection such that it is received again by the transducer. If thesignal strength at point x is S(x) then:

    b(x)S(x) (0≦b(x)≦1)

is the fraction of signal which, after further attenuation on the returntrip, arrives back at the transducer. If it is assumed that theattenuation properties of the tissue are directionally independent, thenthe signal strength, S_(r), received at the transducer after having beenscattered once at point x is given by ##EQU3## where S measures theamplitude of the pulse produced by the transducer and the factor of 2 isdue to attenuation taking place during both the outward and return trip.Since the velocity of sound is assumed to be constant x=ct/2 where t isthe time delay between transmission and reception.

In the case of a B-scan, that is a set of A-scans in a y directiontransverse to the A-scan or x direction, a and b may depend on both xand y. It should be appreciated here that the term "a y direction" isnot intended to limit the transverse direction to a single value butrather to allow it to adopt a plurality of values as may be the case,for example, in the so-called "sector" sweep where the y-directionvaries over a small range, or in a more fundamental variation, usinginternal probes, where the sweep may extend over 360° . The B-scan maybe produced from a transducer comprising an array of transducer elementsand electronically scanning the output signals, or by displacing asignal transducer element along the y direction to provide a set ofA-scan images to make up the B-scan. The quantity b(x,y) is a measure ofbackscatter, it may include a depth dependence to take account of theeffect of spherical spreading of the backscatter pulse before arrival atthe transducer. Moreover, whenever in this specification reference ismade to a signal it will be appreciated that this is to be understood,where appropriate, to include a reference to a function of the signal.

The received signal is amplified by introducing a time-dependent gain inan attempt to compensate for the effects of the exponential factor inthe equation for S_(r) (x). Typically this compensation is achieved bythe equivalent of multiplying the received signal by an exponentialfunction of t (and hence x). The same compensation is applied to eachA-scan line in a B-scan with no account being taken for attenuationvariation in either x or y directions. If S_(d) (x) is this amplifiedsignal then ##EQU4## where A is a constant. The ability to vary thevalue of A is the only adjustment to the time gain that can be made. Theeffects of shadowing and enhancement of the ultrasonic images occurbecause the value of A cannot always be chosen so that Ax is a goodapproximation to ##EQU5## for example, taking a single A-scan so thatthe variable y can be neglected in the equation for S_(d) (x,y) andchoosing units so that S=1, then given a sample with constantbackscatter b(x)=b₀ and constant attenuation a(x)=a₀, then the equationfor S_(d) (x,y) gives

    ∫S.sub.d (x)=b.sub.0 exp(-2a.sub.0 x)exp(2Ax)

if, by adjusting the time gain, A is chosen to equal a₀ then S_(d)(x)=b₀ to give a good representation of the backscatter map. Now, if thetime gain is left fixed, but the attenuation a(x) is:

a₁, if x₁ <x<x₂

a₀, otherwise

This can be considered to be due to the presence of an object whichspans the region from x₁ to x₂. Substituting into the equation for S_(d)(x,y) gives the results that S_(d) (x) is:

b₀, if x<x₁

b₀ exp(-(a₁ -a₀) (x-x₁)) , if x₁ ≦x≦x₂

b₀ exp (-(a₁ -a₀) (x₂ -x₁)) , if x_(2<x) If a 1>a0 then the signal forx>x2 is less than b0 giving a shadow behind the object. This isillustrated in FIG. 2 in which the decay in the value of S_(d) (x)between x, and x₂, represented by slope 11, is followed by a continuouslow value 12 independently of any other attenuation feature.

On the other hand, if a₁ <a₀ then the signal is greater than b₀ for x>x₂which gives an enhancement behind the object as shown in FIG. 3 wherethe rising slope 13 is followed, at higher values of x, by an increasedvalue of S_(d) (x),

In the equation for S_(d) (x,y) tissue attenuation and backscatter arespecified by the quantities a(x,y) and b(x,y) respectively. The imagepresented to the operator, represented by S_(d) (x,y), depends on botha(x,y) and b(x,y). It also depends on the strength of the transmittedpulse S and the time gain, which is represented by the constant A.

Since in practice the scanner does not separate out the effects ofattenuation and backscatter (equivalent to obtaining both a(x,y) andb(x,y)) the only information available is the ultrasound image, S_(d)(x,y). From the equation for S_(d) (x,y) it can be seen that performingthis task is mathematically equivalent to solving one equation for twounknowns. There is no unique solution to this problem but a solution canbe obtained if additional assumptions can be made so that the problem isreduced to solving one equation with a single unknown.

The use of a fixed time gain, as used in most existing equipment, can beconsidered to be based on an assumption that a(x,y) is constant.Automatic time gain algorithms make different assumptions such as, forexample, assuming that b(x,y) is constant. The present invention isbased on the proposition that these assumptions oversimplify the caseand, worse, introduce their own errors. Instead a relationship betweenattenuation a(x) and backscatter b (x) is utilized. Taking the simplecase of b(x)=k a(x) gives: ##EQU6## so that ##EQU7## from this it can beseen that ##EQU8##

Thus, by dividing the received signal by an integral of the receivedsignal over a range from the point of interest onwards (in practice thiswill be a finite range even though the upper limit is expressed as ∞ inthe above expressions) the component dependent on backscatter b(x) iseffectively compensated. This result is arrived at by the assumptionthat the relation between a(x) and b(x) is a simple linear one. Inpractice, of course the relationship will almost certainly be rathermore complex so that, as a general expression, the signal used fordisplay can be represented by S(x) where ##EQU9## and where S(x) is afunction of the received signal, and ##EQU10##

FIG. 4 illustrates in block diagram form the major components of anultrasonic scanning system having means for compensating for imageartifacts. The system shown comprises a transducer 15, which in practicecomprises an array of transducer elements for transmitting and receivingfocused ultrasonic pulses into a body (not shown) under investigation.Circuits for generating the transmitted pulse, and for detecting theecho signal, including filtering, pulse shaping and other processing ofthe signal are not shown since these are conventional on knowncommercially available ultrasonic scanners.

The signal s(t) from the transducer is fed to preprocessing circuits 16,for example for applying a time dependent gain, again as known inconventional scanners to generate a signal S s(t)!which is a function ofthe received signal. This signal is then transmitted both to a delaycircuit 17 or temporary store and to an integrator 18 operable to sumthe values of S s(t)!(or S(t)), for each time instant t from receptionof an echo pulse to a selected end point to form the signal ##EQU11##

The thus integrated signal c(x) is then combined with the delayedfunction S(x) to form the quotient ##EQU12## in combination circuit 19,and signal S(x) is fed to a scan converter and display device 20 such asa video monitor. As discussed above the signal thus processed issubstantially free from shadowing and enhancement effects withoutsuffering from any other image degradation as a result of the furtherprocessing, allowing the clinician greater scope for interpretation ofthe images without the potential masking effects of the image artifactsremoved by the signal processing.

Although described in the context of an ultrasonic scanner as such, theinvention may also be embodied as a signal processing accessory for usein connection with existing ultrasonic scanners by extracting the signalS(t) from the scanner, processing it as described, and reintroducing theprocessed signals to the scanner for display.

Furthermore, although described strictly in relation to ultrasonicechoscopy in which echo amplitude is the modified parameter in thereceived signal, the above-described techniques of the present inventionmay also be applied to pulsed doppler or colour doppler techniques.

I claim:
 1. An apparatus for use in association with an ultrasoundscanner of a type comprising:at least one transducer, means forgenerating a pulse signal to be transmitted by the transducer, whichtransducer receives an echo signal after reflection of the transmittedpulse signal within an object under examination to generate a receivedsignal for producing an A-scan line which, in association with signalsconstituting other A-scan lines forms a B-scan signal of said objectunder examination, the apparatus comprising: means for receiving theB-scan signal, means for deriving therefrom a compensation signalderived by integration of a function of the received signal over afinite time period, and means for dividing the said function of thereceived signal by the compensation signal to form an output B-scansignal, and output means for delivering said B-scan signal to saidassociated ultrasound scanner for display.
 2. A method of compensatingfor the effects of variable attenuation in an object being scanned by anultrasound pulse echo scanner having ultrasonic pulse transmission meansfor transmitting an ultrasonic pulse signal into an object underexamination, ultrasonic pulse receiver means for receiving echo pulsesreflected from within the object and converting them into a receivedsignal, comprising the steps of forming a compensation signal from thereceived signal by summing values of a function of the received signalover time intervals commencing with each of a plurality of timeinstants, and generating a signal for display by dividing the functionof the received signal by the compensating signal.
 3. The method asdefined in claim 2, in which a signal representing the function of thereceived signal is temporarily stored while the values of thecompensation signal are determined.
 4. A method as defined in claim 3,wherein the received signal is subject to variable time gaincompensation before the compensation signal is derived.
 5. A method asdefined in claim 3, wherein the compensation signal is derived on thebasis that the signal strength of the received signal at any timeinstant after generation is influenced by both attenuation andbackscatter and that there is a relation between backscatter andattenuation effects in the medium through which the pulse istransmitted.
 6. The method as defined in claim 2, in which the receivedsignal is subject to variable time gain compensation before thecompensation signal is derived.
 7. A method as defined in claim 2,wherein the compensation signal is derived on the basis that the signalstrength of the received signal at any time instant after generation isinfluenced by both attenuation and backscatter and that there is arelation between backscatter and attenuation effects in the mediumthrough which the pulse is transmitted.
 8. The method as defined inclaim 2, in which the compensation signal is derived on the basis thatthe signal strength of the received signal at any time instant aftergeneration is influenced by both attenuation and backscatter and thatthere is a relation between backscatter and attenuation effects in themedium through which the pulse is transmitted.
 9. A method as defined inclaim 2, in which the step of forming the compensation signal takes intoaccount a relationship between backscatter and attenuation effects inthe medium through which the pulse is transmitted.
 10. A method ofcompensating for the effects of variable attenuation in an object beingscanned by an ultrasound pulse echo scanner having ultrasonictransmission means for transmitting an ultrasonic pulse signal into anobject under examination, ultrasonic pulse receiver means for receivingecho pulses reflected from within the object and for converting theminto a received signal, comprising the steps of:forming a compensationsignal by summing values of a function of the received signal over time,the compensation signal being derived on the basis that the signalstrength of the received signal at any time instant after generation isinfluenced by both attenuation and backscatter and that there is arelationship between backscatter and attenuation effects in the mediumthrough which the pulse is transmitted, and generating a signal fordisplay by dividing the function of the received signal by thecompensating signal.
 11. An apparatus for producing ultrasound images ofa body under examination, comprising:a transducer, means for generatinga pulse signal to be transmitted by the said transducer, whichtransducer receives an echo signal after reflection within the objectunder examination to generate a received signal, means for applying timevaried gain control to the received signal and for producing from thereceived signal a plurality of sequential successive A-scan signalswhich together constitute a B-scan signal representing the echo signalsreflected from within the object under examination, and means for atleast partly compensating image artifacts in the B-scan signal due tothe time varied gain control applied to the received signal, the saidimage artifact compensating means comprising: means for deriving afunction of the received signal, means for deriving a compensationsignal by integration of the function of the received signal over afinite time period, and means for dividing the compensation signal intothe function of the received signal whereby to form an output signal tobe applied to display means for displaying the image thus produced. 12.The apparatus as defined in claim 11, in which there are providedtemporary storage means for storing a signal representing the functionof the received signal, integration means for integrating the functionof the received signal over a finite time period, and divider means fordividing the function of the received signal by the compensation signalto produce the output signal for application to the display means.
 13. Amethod of operating an ultrasonic imaging system comprising the stepsof:a) transmitting ultrasonic pulses into an object under examination,b) receiving echo pulses reflected from within the object underexamination, c) converting the received echo pulses into a receivedsignal, d) generating a function of the received signal, e) deriving acompensation signal by summing values of the function of the receivedsignal over time intervals commencing with each of a plurality of timeinstants, f) generating a signal for display by dividing thecompensation signal into the function of the received signal, and g)subjecting the received signal to variable time gain compensation beforederiving the compensation signal.